Foster mother rat entrains circadian adrenocortical rhythm in blinded pups.

Free-running patterns of blood corticosterone rhythm were examined between the fourth and eighth postnatal week in blinded pups optically enucleated on day 1. When blinded pups were raised by intact natural mothers, the peak time of corticosterone rhythm appeared near the time of light-dark (LD) transition, which was close to that of mothers, and then free-ran. This phase relation between mothers and pups showed that mothers entrained pups' corticosterone rhythm. When blinded pups born of LD (or DL) mothers were raised by DL (or LD) foster mothers, the corticosterone rhythm in blinded pups was close to that of foster mothers at 4 wk of age, indicating that a foster mother plays a predominant role in entraining the pups' rhythm. When blinded pups were raised by blinded mothers whose rhythm was out of phase of the lighting cycle, pups' rhythm was in phase with that of mothers, showing that lighting cycle did not affect the pups' rhythm. These facts indicate that the mother nursing pups acts as the most predominant entrainer of corticosterone rhythm of the blinded pups.